1. Field of the Invention
The present invention relates to an engine adapted to be mounted on a motor bicycle, and particlularly to such an engine which is reduced in size by decreasing the length of the crank case to make it possible to decrease the wheel base (i.e. the distance from the front wheel to the rear wheel of the motor bicycle). More particularly, it relates to an engine in which the crank shaft, the transmission main shaft and the transmission drive shaft are arranged in a unique fashion.
2. Prior Art Statement
In the engine for a motor bicylce, it is a common practice to arrange a crank shaft and a transmission having a main shaft and a drive shaft within a crank case. In order to improve the production efficiency in assembling the parts, it has been proposed to split the crank case into an upper and lower crank cases so that the crank shaft, the transmission main shaft and the transmission drive shaft are disposed on the split plane between the upper and lower crank cases, as disclosed, for example, by Japanese Patent Laid-Open Publication No. 79020/1984.
However, when the crank shaft and the main and drive shafts of the transmission are disposed in-between the split plane between the upper and lower crank cases, the length of the crank case is increased to pose a problem that the size of the engine 15 increased. The rear arms for carrying the rear wheel is pivoted near the point of the frame of the vehicle body at which the engine is suspended. As the length of the engine is increased, the length of the wheel base is also increased correspondingly.